1. Field of the Invention
The present invention relates to an IPS (In-Plane Switching) liquid crystal display device and, more particularly, to a pattern structure of electrodes and wiring in the pixel region of the IPS liquid crystal display device.
2. Description of the Prior Art
In an IPS liquid crystal display device, a liquid crystal is sealed in a space between a pair of transparent substrates and an electric field substantially parallel to a surface of one of these substrates is applied thereto, whereby liquid crystal molecules are rotated and oriented in a direction horizontal to the substrate surface. The IPS liquid crystal display device can achieve a wide viewing angle by orienting the liquid crystal molecules in a direction horizontal to the substrate surface.
Pixel electrodes and a common electrode which are formed at a given spacing are arranged in a comb-like configuration on the transparent substrate to which the electric field is applied in parallel relation. By applying a voltage between the pixel electrodes and the common electrode, an electric field parallel to the substrate surface is generated. In the IPS liquid crystal display device, therefore, display is constantly viewed only from a direction in which the short axes of the liquid crystal molecules extend so that the viewing angle is extremely widened.
Thus, the IPS liquid crystal display device has the advantage of the extremely wide viewing angle, while having the disadvantage that a screen is tinted in a yellow or blue color when viewed in a tilted fashion from the longitudinal direction of the electrodes in the comb-like configuration. To solve the problem, Japanese Laid-Open Patent Publication No. Hei 10-148826 discloses a structure in which each of the electrodes in the comb-like configuration is flexed, as shown in a plan view of a pixel region of FIG. 1.
Referring to FIG. 1, TFT switching elements 109 are provided at the individual points of intersection of a plurality of signal lines 101 and scan lines 108 which are arranged as a matrix. A pixel electrode 106 and a common electrode 107 are formed in a comb-like and interdigitated configuration. These electrodes are connected to the TFT switching elements 109. The pixel electrode 106, the common electrode 107, and the signal lines 101 are formed in flexed and mutually parallel configuration.
In the respective regions upper and lower than the bent portions of the flexed electrodes, a liquid crystal is tinted in yellow and blue colors when the screen is viewed in a tilted fashion from the longitudinal direction of the flexed electrodes during the application of a voltage. As a result, the color changes relative to the viewing angle are mutually compensated for so that an image with no color change is obtainable.
In the multi-domain IPS liquid crystal display device having the flexed electrode pattern structure, however, outer peripheral configuration (edge pattern) of each of the electrodes in the pixel region thereof is directly linear and is not parallel or orthogonal to the optical axis of a polarizer plate. Consequently, diffracted light having a polarized light component which is not absorbed by an analyzer (polarizer plate) is generated by the outer peripheral portion (edge portion), so that optical leakage occurs. Thus, the liquid crystal display device has the problem that the optical leakage increases dark brightness and degrades contrast.
A description will be given to the problem with reference to FIGS. 2A and 2B. FIG. 2A is a plan view diagrammatically showing the pixel region in the absence of optical leakage. FIG. 2B is a diagrammatical plan view illustrating the state of optical leakage in the pixel region.
As shown in FIG. 2A, a comb-like electrode 102 having a conventional structure is composed of the pixel electrode 106 and the common electrode 107, as shown in FIG. 1, while composing a flexed electrode 103 in conjunction with the signal line 101. The flexed electrodes 103 has a pattern structure in which each of the comb-like electrode 102 and the signal line 101 is flexed. Since the flexed electrode 103 in the pixel region is made of a metal material such as Cr, the portion corresponding to the flexed electrode 103 does not transmit light.
Although dark display (the state in which a voltage is not applied between the pixel electrode 106 and the common electrode 107) is actually performed in the pixel region, optical leakage is observed around the outer peripheral portion of the flexed electrode 103, which serves as a factor causing lower contrast.
Since the electrode edge portion is not parallel or orthogonal to the optical axis 104 in the conventional flexed electrode 3, as shown in FIG. 2B, the diffraction of light by the electrode edge portion causes optical leakage 105 between crossed nicols. As a result, dark brightness increases and contrast deteriorates. Thus, the metal electrode which is not orthogonal or parallel to the optical axis 104 has the problem that the outer peripheral portion thereof generates a polarized light component which is not absorbed by the analyzer as a result of diffraction and increases the dark brightness.
This indicates that an amount of optical leakage increases as a tilt angle θ (θ represents an angle shown in FIG. 6B) between wiring and optical axis approaches 45 degrees but becomes 0 at each of 0 and 90 degrees, as shown in FIG. 3. The graph of FIG. 3 was obtained by experiment by the present inventors et al. As an optimal value of the tilt angle θ, 15 degrees is normally set.